Roll-to-roll non-vacuum deposition of transparent conductive electrodes

ABSTRACT

Methods and devices are provided for improved photovoltaic devices. Non-vacuum deposition of transparent conductive electrodes in a roll-to-roll manufacturing environment is disclosed. In one embodiment, a method is provided for forming a photovoltaic device. The method comprises processing a precursor layer in one or more steps to form a photovoltaic absorber layer; depositing a smoothing layer to fill gaps and depression in the absorber layer to reduce a roughness of the absorber layer; adding an insulating layer over the smooth layer; and forming a web-like layer of conductive material over the insulating layer. By way of nonlimiting example, the web-like layer of conductive material comprises a plurality of carbon nanotubes. In some embodiments, the absorber layer is a group IB-IIIA-VIA absorber layer.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of and claims the prioritybenefit of U.S. patent application Ser. No. 12/396,435, filed Mar. 2,2009, the entire contents of which are fully incorporated herein byreference. U.S. Patent Application claims the benefit of priority toU.S. Provisional Patent Application Ser. No. 61/032,425 filed Feb. 28,2008 and fully incorporated herein by reference for all purposes.

FIELD OF THE INVENTION

The present invention is directed to non-vacuum depositing transparentconductive electrodes (TCE) on large area substrates and morespecifically to non-vacuum TCE deposition in high-throughputroll-to-roll production systems for use in photovoltaics.

BACKGROUND OF THE INVENTION

Solar cells and solar modules convert sunlight into electricity. Theseelectronic devices have been traditionally fabricated using silicon (Si)as a light-absorbing, semiconducting material in a relatively expensiveproduction process. To make solar cells more economically viable, solarcell device architectures have been developed that can inexpensivelymake use of thin-film, light-absorbing semiconductor materials such ascopper-indium-gallium-selenide (CIGS) and the resulting devices areoften referred to as CIGS solar cells.

A central challenge in cost-effectively constructing a large-areaCIGS-based solar cell or module involves reducing processing costs andmaterial costs. In known versions of CIGS solar cells, the transparentelectrode layer and many other layers are deposited by a vacuum-basedprocess over a rigid glass substrate. Typical deposition techniquesinclude co-evaporation, sputtering, chemical vapor deposition, or thelike. The nature of vacuum deposition processes requires equipment thatis generally low throughput and expensive. Vacuum deposition processesare also typically carried out at high temperatures and for extendedtimes. Traditional sputtering or co-evaporation techniques are limitedto line-of-sight and limited-area sources, tending to result in poorsurface coverage and non-uniform three-dimensional distribution of theelements.

High-efficiency thin-film solar cells based on polycrystalline CIGS(copper indium gallium di-selenide, but not excluding any other of theIB, IIIA, VIA elements like e.g. aluminum, and sulfur) are typicallymade with a transparent conductive oxide (TCO) deposited on top of astack containing the CIGSe film, where depending on interconnect schemesome require additional conductive patterns (traces, fingers, grids,lines, bus bars, etc.) to collect the current with minimalelectrical-resistive and optical-shadowing losses. Lowering the cost ofthe deposition of these transparent conductive layers and conductivepatterns is required to minimize the overall cost of the solar panels.

One of the most common techniques used to roll-to-roll deposittransparent conductive electrodes (TCE) is sputter deposition oftransparent conductive oxides (TCO). Unfortunately, for the filmthickness and high vacuum required, sputter deposition is a slow processwith an undesired low throughput/capex ratio. In addition, materialyield is low due to deposition of material onto the chamber walls.Furthermore, temperature control during sputter deposition can limit thethroughput even further, especially when damage of underlyingtemperature-sensitive layers, like e.g. the CIGSe/CdS stack, needs to beprevented. Finally, controlling the large-area uniformity of both theconductivity and transparency of a sputter-deposited TCO is challenging.

In an attempt to lower manufacturing cost, solution-deposition oftransparent conductive layers has been investigated by others. Oneexample uses high-temperature sintering of solution-deposited metaloxide particles and subsequently these particles are sintered to a denselayer. A huge disadvantage of this method is the temperature required toget dense layers. Temperatures required to sinter these layers damagethe underlying films as used in thin-film PV.

Other examples use individually grown carbon nanotubes or metallicnanowires that are solution-deposited and where the layer ismechanically stabilized by organic additives allowing processing attemperatures that prevent damage to the underlying layers. One bigdisadvantage of this approach is the limited lateral electricalconductivity that can be accomplished without loosing too much opticaltransparency. Additionally, since the amount of organics required tomechanically stabilize these layers is considerable, a bi-continuouspercolating conductive network inside an electrically-insulating networkis created. The two-phase nature of this approach limits the contactarea that can be achieved at both interfaces when sandwiched betweenother (semi)conductive layers. This limited contact area affects thecontact resistance in a negative way. Additionally, a possible mismatchin the coefficients of thermal expansion of both the conductive andinsulating materials might impact the temperature dependence andreliability of the overall conductivity of the bi-continuous network ina negative way.

A third approach mixes organic nanotubes with doped conjugated polymersto increase the lateral conductivity compared to nanotubes only.Typically, the chemical stability of organic materials is photosensitiveand the conductivity is smaller than of inorganic materials.

Summarizing, the major challenges to uniformly deposit ahighly-conductive, highly-transparent (for AM1.0 or AM1.5),chemically-stable, reliable thin layer onto a large area without use ofvacuum and high temperature.

Due to the aforementioned issues, improved techniques may be used forreducing processing costs and material costs. Improvements may be madeto increase the throughput of existing manufacturing processes anddecrease the cost associated with CIGS based solar devices. Thedecreased cost and increased production throughput should increasemarket penetration and commercial adoption of such products.

SUMMARY OF THE INVENTION

Embodiments of the present invention address at least some of thedrawbacks set forth above. It should be understood that at least someembodiments of the present invention may be applicable to any type ofsolar cell, whether they are rigid or flexible in nature or the type ofmaterial used in the absorber layer. Embodiments of the presentinvention may be adaptable for roll-to-roll and/or batch manufacturingprocesses. In one embodiment, low cost transparent conductor (TC) isprovided that will work on printed & rapid thermally processed CIGS,CIGSS, or other absorber layer to reduce or eliminate the amount ofmaterial that needs to be sputtered or chemical vapor deposited. Atleast some of these and other objectives described herein will be met byvarious embodiments of the present invention.

Embodiments of the present invention may address the challenges involvedfabricating such transparent conductive electrodes in a cost-effectivemanner. In one embodiment, a thin, transparent, conductive sheet or rollis transferred from a sacrificial material to a partially completedsolar cell stack via standard transfer (adhesion and release)technology. The thin, transparent, conductive sheet or roll mightconsist of high-temperature, sintered, transparent, conductive oxideparticles, and the sacrificial material might be aluminum foil. Thetransparent conductive sheet or roll might also consist of blockcopolymers filled with conductive material. The block copolymers havebeen solution-deposited onto a sacrificial material, followed by fillingthe pores with conductive material. This conductive material might bedeposited into the pores of the block copolymers via electro(less)deposition Some alternatives include depositing an ultrathin layer ofsilver and then patterning it lithographically. Various methods ofself-assembly have been described which have the potential to arrangesmall particles in usefully ordered arrays. In one embodiment, asolution coatable transparent conductor layer for CIGS or CIGSS solarcells is provided to simplify cell manufacturing. It should beunderstood that a TCE layer may in one embodiment be that taught in U.S.patent application Ser. No. 10/338,079 filed Jan. 6, 2003 and fullyincorporated herein by reference for all purposes.

In one embodiment of the present invention, the transparent electrode ofa thin-film CIGS solar cell is replaced in part or completely by a sheetof carbon nanotubes. This sheet might be grown by catalytic thermalchemical vapor deposition (CVD) and pulled into a dense, transparent,conductive sheet of oriented multiwall carbon nanotubes from a lessdense forest of multiwall carbon nanotubes. This sheet is subsequentlylaminated onto the underlying layers to allow formation of a thin-filmCIGS solar cell.

In another embodiment of the present invention a conductive, transparentlayer is grown onto the partially completed stack of layers to form athin-film CIGS solar cell by use of solution-deposition ofblock-copolymers, followed by filling the block-copolymers withconductive material.

In yet another embodiment of the present invention graphene is used as atransparent conductive layer. Graphene is single sheet graphite. It isextremely conductive in the x-y direction and extremely transparent inthe z direction due to its 2D structure. Graphene sheets can beexfoliated from graphite. One method of exfoliation is with tape.Deposition could be done with solution coating by dispersing exfoliatedgraphene into solution followed by coating. Deposition could also bedone via transfer via standard adhesion and release layer technology.Finally another method could be to use a tape that can be permanentlyleft on top of the solar cell.

In one embodiment of the present invention, a method is provided forforming a photovoltaic device. The method comprises processing aprecursor layer in one or more steps to form a photovoltaic absorberlayer; depositing a smoothing layer to fill gaps and depression in theabsorber layer to reduce a roughness of the absorber layer; adding aninsulating layer over the smooth layer; and forming a web-like layer ofconductive material over the insulating layer. By way of nonlimitingexample, the web-like layer of conductive material comprises a pluralityof carbon nanotubes. In some embodiments, the absorber layer is a groupIB-IIIA-VIA absorber layer.

In one embodiment of the present invention, the transparent electrode ofa solar cell is replaced in part by a carbon nanotubes coating that isless than one monolayer of tubes. In this embodiment, to address anyroughness of underlying CIGS/ZnO(i), thicker insulator (either i-ZnO orother material) may be used. One can imagine that the ZnO(i) would notbe able to completely cover the rough CIGS or CIGSS surface and thatthese tubes can find their way anywhere and seem to be a continuousnetwork of highly conducting wires. The evaporation of Ni,Al fingerswill go into the CIGS layer and create a shunt since the tube network isporous and the ZnO(i) is presumably spotty. Using a thicker insulator orone of the other solutions described herein will reduce shunting inrough absorber layers using web-like transparent conductors.

In another embodiment of the present invention, the method comprisesprocessing a precursor layer in one or more steps to form a photovoltaicabsorber layer; depositing a smoothing layer to fill gaps and depressionin the absorber layer to reduce a roughness of the absorber layer;adding an insulating layer over the smoothing layer; and forming aweb-like layer of conductive material over the insulating layer.

In yet another embodiment of the present invention, a method comprisesprocessing a precursor layer in one or more steps to form a photovoltaicabsorber layer; and depositing a smoothing, insulating layer to fillgaps and depression in the absorber layer to reduce a roughness of theabsorber layer.

It should be understood that for any of the embodiments herein, thefollowing may optionally also apply. Optionally, the smoothing insulatoris sufficient to cover all peaks of the rough absorber, but sufficientlythin to allow electrons to pass out of the absorber layer. Optionally,the smoothing insulator conformally covers all peaks in the roughabsorber. Optionally, the method includes further comprising creatingthe absorber layer by processing the precursor layer into a solid filmand then thermally reacting the solid film in an atmosphere containingat least an element of Group VIA of the Periodic Table to form thephotovoltaic absorber layer. Optionally, creating the absorber layer bythermal reaction of the precursor layer in an atmosphere containing atleast an element of Group VIA of the Periodic Table to form thephotovoltaic absorber layer. Optionally, Group IB and/or IIIA hydroxidecomprises indium-gallium hydroxide and used as a precursor material forthe absorber layer. Optionally, the web material may be solution coatedonto the target surface. Examples of solution deposition methods mayinclude at least one method from the group comprising: wet coating,spray coating, spin coating, doctor blade coating, contact printing, topfeed reverse printing, bottom feed reverse printing, nozzle feed reverseprinting, gravure printing, microgravure printing, reverse microgravureprinting, comma direct printing, roller coating, slot die coating,meyerbar coating, lip direct coating, dual lip direct coating, capillarycoating, ink-jet printing, jet deposition, spray deposition, aerosolspray deposition, dip coating, web coating, microgravure web coating, orcombinations thereof. These applications of carbon nanotubes provide newavenues to lower costs, better durability, better thermal stability, andhigher efficiencies. Of course, other non-solution based techniques mayalso be used.

In yet another embodiment of the present invention, a method is providedcomprising of processing a precursor layer in one or more steps to forma photovoltaic absorber layer; and depositing a conformal insulatinglayer over the gaps and depression in the absorber layer to minimizeshunting due to roughness of the absorber layer.

One embodiment of the present invention uses solution-depositionfollowed by subsequent low-temperature processing, is to be able toformulate an ink (slurry, paste, dispersion, emulsion, paint, solution)that allows the following. (1) One single or multiplesolution-deposition steps of (precursors of) conductive materials ontoan underlying layer without the negative influenceelectrically-insulating organic additives might have on the contactresistance to the underlying layer (and overlying layer), and volumeresistivity within the bulk of the transparent conductive layer. (2)Limiting the sintering time and temperature. (3) Enhancing conductivitybetween the individual, discrete, electrically conductive particles(rods, wires, tetrapods). (4) Minimizing the impact of the wettabilityof the underlying layer and the formulation of the ink on the morphologyof the as-deposited ink where the morphology impacts both the contactresistance and volume resistivity. (5) Limiting the negative impact of apossible CTE-mismatch within the layer on volume resistivity withtemperature and time. (6) Allowing for chemically, mechanically, andelectrically stable layers for a reliable stable product. In order toovercome the difficulties with typical materials used forsolution-deposition, new materials and/or methods are required.

A further understanding of the nature and advantages of the inventionwill become apparent by reference to the remaining portions of thespecification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a photovoltaic device accordingto one embodiment of the present invention.

FIG. 2 shows an image of nanotubes according to one embodiment of thepresent invention.

FIGS. 3A and 3B show cross-sectional and top down views of photovoltaicdevices according to embodiments of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed. It may be notedthat, as used in the specification and the appended claims, the singularforms “a”, “an” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, reference to “a material”may include mixtures of materials, reference to “a compound” may includemultiple compounds, and the like. References cited herein are herebyincorporated by reference in their entirety, except to the extent thatthey conflict with teachings explicitly set forth in this specification.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

“Optional” or “optionally” means that the subsequently describedcircumstance may or may not occur, so that the description includesinstances where the circumstance occurs and instances where it does not.For example, if a device optionally contains a feature for ananti-reflective film, this means that the anti-reflective film featuremay or may not be present, and, thus, the description includes bothstructures wherein a device possesses the anti-reflective film featureand structures wherein the anti-reflective film feature is not present.

Photovoltaic Device

Referring now to FIG. 1, one example of a photovoltaic device is shown.The device 50 includes a base substrate 52, an optional adhesion layer53, a base or back electrode 54, a p-type absorber layer 56, an n-typesemiconductor thin film 58 and a transparent electrode 60. By way ofexample, the base substrate 52 may be made of a metal foil, a polymersuch as polyimides (PI), polyamides, polyetheretherketone (PEEK),Polyethersulfone (PES), polyetherimide (PEI), polyethylene naphtalate(PEN), Polyester (PET), related polymers, a metallized plastic, and/orcombination of the above and/or similar materials. By way of nonlimitingexample, related polymers include those with similar structural and/orfunctional properties and/or material attributes. The base electrode 54is made of an electrically conductive material. By way of example, thebase electrode 54 may be of a metal layer whose thickness may beselected from the range of about 0.1 micron to about 25 microns. Anoptional intermediate layer 53 may be incorporated between the electrode54 and the substrate 52. The transparent electrode 60 may include atransparent conductive layer 59 and a layer of metal (e.g., Al, Ag, Cu,or Ni) fingers 61 to reduce sheet resistance. Optionally, the layer 53may be a diffusion barrier layer to prevent diffusion of materialbetween the substrate 52 and the electrode 54. The diffusion barrierlayer 53 may be a conductive layer or it may be an electricallynonconductive layer. As nonlimiting examples, the layer 53 may becomposed of any of a variety of materials, including but not limited tochromium, vanadium, tungsten, and glass, or compounds such as nitrides(including tantalum nitride, tungsten nitride, titanium nitride, siliconnitride, zirconium nitride, and/or hafnium nitride), oxides, carbides,and/or any single or multiple combination of the foregoing. Although notlimited to the following, the thickness of this layer can range from 10nm to 50 nm. In some embodiments, the layer may be from 10 nm to 30 nm.Optionally, an interfacial layer may be located above the electrode 54and be comprised of a material such as including but not limited tochromium, vanadium, tungsten, and glass, or compounds such as nitrides(including tantalum nitride, tungsten nitride, titanium nitride, siliconnitride, zirconium nitride, and/or hafnium nitride), oxides, carbides,and/or any single or multiple combination of the foregoing. Thetransparent conductive layer 59 may be inorganic, e.g., a transparentconductive oxide (TCO) such as but not limited to indium tin oxide(ITO), fluorinated indium tin oxide, zinc oxide (ZnO), aluminum dopedzinc oxide (AZO), gallium doped zinc oxide (GZO), boron doped zinc oxide(BZO).

Aluminum and molybdenum can and often do inter-diffuse into one another,with deleterious electronic and/or optoelectronic effects on the device50. To inhibit such inter-diffusion, an intermediate, interfacial layer53 may be incorporated between the aluminum foil substrate 52 andmolybdenum base electrode 54. The interfacial layer may be composed ofany of a variety of materials, including but not limited to chromium,vanadium, tungsten, and glass, or compounds such as nitrides (includingbut not limited to titanium nitride, tantalum nitride, tungsten nitride,hafnium nitride, niobium nitride, zirconium nitride vanadium nitride,silicon nitride, or molybdenum nitride), oxynitrides (including but notlimited to oxynitrides of Ti, Ta, V, W, Si, Zr, Nb, Hf, or Mo), oxides,and/or carbides. The material may be selected to be an electricallyconductive material. In one embodiment, the materials selected from theaforementioned may be those that are electrically conductive diffusionbarriers. The thickness of this layer can range from 10 nm to 50 nm orfrom 10 nm to 30 nm. Optionally, the thickness may be in the range ofabout 50 nm to about 1000 nm. Optionally, the thickness may be in therange of about 100 nm to about 750 nm. Optionally, the thickness may bein the range of about 100 nm to about 500 nm. Optionally, the thicknessmay be in the range of about 110 nm to about 300 nm. In one embodiment,the thickness of the layer 53 is at least 100 nm or more. In anotherembodiment, the thickness of the layer 53 is at least 150 nm or more. Inone embodiment, the thickness of the layer 53 is at least 200 nm ormore. Optionally, some embodiments may include another layer such as butnot limited to an aluminum layer above the layer 53 and below the baseelectrode layer 54. This layer may be thicker than the layer 53.Optionally, it may be the same thickness or thinner than the layer 53.This layer 53 may be placed on one or optionally both sides of thealuminum foil (shown as layer 55 in phantom in FIG. 1).

If barrier layers are on both sides of the aluminum foil, it should beunderstood that the protective layers may be of the same material orthey may optionally be different materials from the aforementionedmaterials. The bottom protective layer 55 may be any of the materials.Optionally, some embodiments may include another layer 57 such as butnot limited to an aluminum layer above the layer 55 and below thealuminum foil 52. This layer 57 may be thicker than the layer 53 (or thelayer 54). Optionally, it may be the same thickness or thinner than thelayer 53 (or the layer 54). Although not limited to the following, thislayer 57 may be comprised of one or more of the following: Mo, Cu, Ag,Al, Ta, Ni, Cr, NiCr, or steel. Some embodiments may optionally havemore than one layer between the protective layer 55 and the aluminumfoil 52. Optionally, the material for the layer 55 may be anelectrically insulating material such as but not limited to an oxide,alumina, or similar materials. For any of the embodiments herein, thelayer 55 may be used with or without the layer 57.

The nascent absorber layer 56 may include material containing elementsof groups IB, IIIA, and (optionally) VIA. Optionally, the absorber layercopper (Cu) is the group IB element, Gallium (Ga) and/or Indium (In)and/or Aluminum may be the group IIIA elements and Selenium (Se) and/orSulfur (S) as group VIA elements. The group VIA element may beincorporated into the nascent absorber layer 56 when it is initiallysolution deposited or during subsequent processing to form a finalabsorber layer from the nascent absorber layer 56. The nascent absorberlayer 56 may be about 1000 nm thick when deposited. Subsequent rapidthermal processing and incorporation of group VIA elements may changethe morphology of the resulting absorber layer such that it increases inthickness (e.g., to about twice as much as the nascent layer thicknessunder some circumstances).

Fabrication of the absorber layer on the aluminum foil substrate 52 isrelatively straightforward. First, the nascent absorber layer isdeposited on the substrate 52 either directly on the aluminum or on anuppermost layer such as the electrode 54. By way of example, and withoutloss of generality, the nascent absorber layer may be deposited in theform of a film of a solution-based precursor material containingnanoparticles that include one or more elements of groups IB, IIIA and(optionally) VIA. Examples of such films of such solution-based printingtechniques are described e.g., in commonly-assigned U.S. patentapplication Ser. No. 10/782,017, entitled “SOLUTION-BASED FABRICATION OFPHOTOVOLTAIC CELL” and also in PCT Publication WO 02/084708, entitled“METHOD OF FORMING SEMICONDUCTOR COMPOUND FILM FOR FABRICATION OFELECTRONIC DEVICE AND FILM PRODUCED BY SAME” the disclosures of both ofwhich are incorporated herein by reference.

In the present embodiment, layer 58 may be an n-type semiconductor thinfilm that serves as a junction partner between the compound film and thetransparent conducting layer 59. By way of example, the n-typesemiconductor thin film 58 (sometimes referred to as a junction partnerlayer) may include inorganic materials such as cadmium sulfide (CdS),zinc sulfide (ZnS), zinc hydroxide, zinc selenide (ZnSe), n-type organicmaterials, or some combination of two or more of these or similarmaterials, or organic materials such as n-type polymers and/or smallmolecules. Layers of these materials may be deposited, e.g., by chemicalbath deposition (CBD) and/or chemical surface deposition (and/or relatedmethods), to a thickness ranging from about 2 nm to about 1000 nm, morepreferably from about 5 nm to about 500 nm, and most preferably fromabout 10 nm to about 300 nm. This may also be configured for use in acontinuous roll-to-roll and/or segmented roll-to-roll and/or a batchmode system.

The transparent conductive layer 59 may be inorganic, e.g., atransparent conductive oxide (TCO) such as but not limited to indium tinoxide (ITO), fluorinated indium tin oxide, zinc oxide (ZnO) or aluminumdoped zinc oxide, or a related material, which can be deposited usingany of a variety of means including but not limited to sputtering,evaporation, chemical bath deposition (CBD), electroplating, sol-gelbased coating, spray coating, chemical vapor deposition (CVD), physicalvapor deposition (PVD), atomic layer deposition (ALD), and the like.Alternatively, the transparent conductive layer may include atransparent conductive polymeric layer, e.g. a transparent layer ofdoped PEDOT (Poly-3,4-Ethylenedioxythiophene), carbon nanotubes orrelated structures, or other transparent organic materials, eithersingly or in combination, which can be deposited using spin, dip, orspray coating, and the like or using any of various vapor depositiontechniques. Optionally, it should be understood that intrinsic(non-conductive) i-ZnO may be used between CdS and Al-doped ZnO.Combinations of inorganic and organic materials can also be used to forma hybrid transparent conductive layer. Thus, the layer 59 may optionallybe an organic (polymeric or a mixed polymeric-molecular) or a hybrid(organic-inorganic) material. Examples of such a transparent conductivelayer are described e.g., in commonly-assigned US Patent ApplicationPublication Number 20040187317, which is incorporated herein byreference.

Those of skill in the art will be able to devise variations on the aboveembodiments that are within the scope of these teachings. For example,it is noted that in embodiments of the present invention, portions ofthe IB-IIIA precursor layers (or certain sub-layers of the precursorlayers or other layers in the stack) may be deposited using techniquesother than particle-based inks For example precursor layers orconstituent sub-layers may be deposited using any of a variety ofalternative deposition techniques including but not limited tosolution-deposition of spherical nanopowder-based inks, vapor depositiontechniques such as ALD, evaporation, sputtering, CVD, PVD,electroplating and the like.

Solution Deposited Transparent Conductors

Referring now to FIG. 2, yet another embodiment of the present inventionwill now be described. This embodiment of the invention shows that thematerial in the transparent electrode layer 59 may be replaced withnon-traditional transparent electrode material such as, but not limitedto, carbon nanotubes 66 as shown in the top down view of FIG. 2.

Carbon nanotubes (CNTs) 66 and/or other conductive fibrous materials canprovide electrical conductance at packing densities that provide partialoptical transparency. Optionally, the layer has very little absorbancein the spectral range from about 400 nm to about 1100 nm. As seen inFIG. 2, the carbon nanotubes 66 when deposited resemble a fibrous orweb-like covering. It should be understood that the fibrous conductormay be used with or without i-ZnO. Besides carbon nanotubes, othersuitable materials may also be used for a printable transparentconductor. Some embodiments may comprise of metal-based nanoassembledlayers that are suitable as transparent conductors. These materials mayalso be fibrous in nature.

A spectrum of techniques and device constructions may be used forapplying these materials to the fabrication of low-cost, long-livedthin-film solar cells, in particular cells constructed on low-cost metalfoils, including cells fashioned in an emitter wrap-through structure.Examples of suitable solution deposition methods may include at leastone method from the group comprising: wet coating, spray coating, spincoating, doctor blade coating, contact printing, top feed reverseprinting, bottom feed reverse printing, nozzle feed reverse printing,gravure printing, microgravure printing, reverse microgravure printing,comma direct printing, roller coating, slot die coating, meyerbarcoating, lip direct coating, dual lip direct coating, capillary coating,ink-jet printing, jet deposition, spray deposition, aerosol spraydeposition, dip coating, web coating, microgravure web coating, orcombinations thereof. These applications of carbon nanotubes provide newavenues to lower costs, better durability, better thermal stability, andhigher efficiencies. Of course, other non-solution based techniques mayalso be used.

Although promising, the work on replacing the known transparentelectrode is not without challenges in terms of process ease or expense.The cell performance may be worse (low shunt resistance) when the carbonnanotubes layer 66 is used in conjunction with printable CIGS on glasswith evaporated selenium/RTP selenization and thin i-ZnO. Upon furtherinvestigation, one reason for the shunting is because the absorber layer56 is too rough to be protected by the i-ZnO and the electricalproperties may not be suited for further protection like those of theZnO:Al are.

To address some of these issues, one embodiment of the present inventionmay address the issue by designing a smoother interface with thetransparent conductor layer. This may involve adjusting or modifying thesubstrate on which the absorber layer 56 is formed or other techniques.By modifying the underlying layer, this results in an absorber layer 56that is smoother without actually adding additional surface treatment tothe absorber layer 56 itself. If the absorber layer 56 is sufficientlysmooth, then the shunting issue would be minimized and a number ofvarious materials may be used to provide the insulation desired at thatinterface. Examples of layers that can be deposited before the layer ofcarbon nanotubes layer 66 in this case are insulating polymers depositedby standard solution coatings, polyelectrolytes deposited via dipcasting or a bath technique, sol gels resulting in inorganic ormetal-organic layers, or similar materials.

Alternative Embodiments

Note that for either the smooth or rough case that the insulatormaterial could also be the same material as the pottant/encapsulatingmaterial, or a material that provides better adhesion for the pottant,or a material that protects the cell from the potent. This also applieseven if the embodiments use i-ZnO on smooth CIGS because the CNTs maydesire a binder to give them extra stability. In this case that bindercan be the pottant/encapsulating material, or a material that providesbetter adhesion for the pottant, EVA and related compounds, arecandidates. Binders include millions that are commercially available.

Optionally, the use of web-like layer of conductive material such ascarbon nanotubes may replace both the ZnO:Al and i-ZnO layers in a solarcell. Embodiments of the present invention may use a solution coated CNTtransparent conductor without shunting by using smooth instead of roughabsorber layers. Optionally the present invention may use a roughabsorber layer that is treated with other layers to minimize shunting.

Embodiments of the present invention also contemplate the facilitationof EWT style cells with group IB-IIIA-VIA absorber layers as seen inFIG. 3A. If the ZnO:Al cannot be eliminated, it can be made thinner bycombining it with the web-like transparent conductor layer.

Optionally, instead of ZnO:Al, in the one embodiment of the presentinvention the entire layer may be replaced by a layer of CNT or otherfibrous or porous layer.

Optionally, the web-like transparent conductor layer is used inconjunction with ZnO:Al to make thinner ZnO:Al layers. By way ofnonlimiting example, carbon nanotubes as a first layer and very thinmetal oxide coating as an overlayer that provides mechanical cohesion(e.g. as a binder) of the underlying nanotube coating and provides topsurface chemical durability for long service life. The thickness of theZnO:Al layer may be in the range of about 50 nm to about 500 nm.

Optionally, instead of all ZnO, the entire layer may be replaced by alayer of CNT or other fibrous or porous layer.

Optionally, on top of ZnO, the entire layer may be replaced by a layerof CNT or other fibrous or porous layer as the primary or soletransparent conductor for lateral conduction, whether assisted or not bya conductive (metallic) grid

Optionally, in between ZnO:Al and i-ZnO layers, the layer may bereplaced by a layer of CNT or other fibrous or porous layer.

Optionally, in between multiple ZnO:Al layers, the web-like transparentconductor layer may be used.

Optionally, in between multiple i-ZnO layers, the web-like transparentconductor layer.

Optionally, in place of CdS and ZnO, a web-like transparent conductormay be used. The web-like transparent conductor may be bound by asuitable binder. A material such as ZnS, CdS, or ZnO may be used toprovide a matrix wherein the web-like transparent conductor is used toimprove the conductivity of the surrounding material (ZnS, CdS, or ZnO)used as the junction partner with the absorber layer.

Optionally, in place of fingers, a web-like transparent conductor layermay be used.

Optionally, in place of bus bar, a web-like transparent conductor layermay be used.

Optionally, in conjunction with a binder to provide stability to thelayer. The binder may be a conductive binder. The binder may be amaterial that is suitable as a junction partner with the absorber layer.

Optionally, using the same binder as the insulator and/or adhesiverequired to laminate and seal the strings/modules.

Optionally, using the same binder as a thin layer between the CNT layerand CIGS/(CdS) to prevent shunting in place of i-ZnO.

Optionally, in conjunction with a conformal deposition of insulatingmaterial to prevent shunting where usually the roughness of the CIGSprevents continuous coverage of a sputtered layer.

Optionally, in conjunction with ALD deposited insulator, e.g. using anALD top coating to provide both a binder function to the underlyingnanotubes and an environmental protection function vis-à-vis cellstability in the field

Optionally, in conjunction with an insulating binder or other overlayerto protect the device whereby electrical contact can only be made bypenetrating the protective layer (with via for example)

Optionally, a web-like transparent conductor layer may be used with anemitter wrap through (EWT) type solar cell. Further details of such anembodiment may be found with reference to FIG. 4. If used with an EWTsolar, the following may apply:

a. A web-like transparent conductor layer may be used as the conductivetransparent top coating of an emitter wrap through (EWT) cell structure,where a sheet resistance of about 10- about 1000 ohm/sq is used,optionally about 40- about 200 ohm/sq, or optionally about 50- about 100ohm/sq

b. A web-like transparent conductor layer may be used as a conductivetransparent top coating deposited on a EWT cell stack after theformation of insulated holes, serving thereby to provide both lateralsheet conductance and through-hole conductance.

c. A web-like transparent conductor layer may be used as a conductivetransparent element providing better cell durability as a result ofbetter thermal expansion matching and better adhesion to EWT materialsof construction.

Note that an additional advantage with EWT might be related to thesolution processibility in that hole punching might cause less damage tothem or that they can be applied after the hole and used also as thewrap through conductor.

Optionally, a web-like transparent conductor layer may be used on CIGScells made on metal foil (thus giving them the smoothness desired forcomplete insulator coverage)

Optionally, a web-like transparent conductor layer may be used to makelayers or lines <0.01 ohm/sq

Optionally, a web-like transparent conductor layer may be used to makelayers or lines ˜50 ohm/sq

Optionally, a web-like transparent conductor layer may be used to makelayers or lines ˜200 ohm/sq for use with EWT technology

Optionally, a web-like transparent conductor layer may be used to makelayers or lines >500 ohm/sq (see static claim)

Optionally, a web-like transparent conductor layer may be used to makeanti-static coatings for solar modules

Optionally, a web-like transparent conductor layer may be used as viafiller material in EWT cells

Optionally, a web-like transparent conductor layer may be used as atransparent conductive element in a thin-film solar cell comprisingfilms fabricated from nanoparticulate materials

a. wherein the nanoparticulate materials are deposited in layers

b. wherein the nanoparticulate materials are sintered into solid films

c. wherein the nanoparticulate materials are precursors to groupIB-IIIA-VIA films, CuInSe2 films, and/or alloys of CIS comprising Gaand/or S

d. wherein the cell constructed as an emitter wrap through design

e. wherein the comprising a flexible substrate

f. wherein the comprising a metal foil (substrate)

Although ZnO:Al and i-ZnO are used above, it should be understood thattheir use is purely exemplary and more generally speaking, various“conductive TCO” and “insulating TCO” are suitable.

TCE Qualities

Although not limited to the following, the conductivity desired forsolution deposited transparent conductor on solar cells may be on theorder of about 100 Ω/sq., optionally not more than about 200 Ω/sq., andoptionally as low as about 10 Ω/sq or less, with very little absorbancein the spectral range from about 400 to about 1100 nm (the differencebetween actual and 100% transmittance should ideally be solelyreflectance, which for the realized indices of refraction of TCO filmsis around 10-15%). ITO films, especially if deposited at temperatures ofa few hundred C, can provide 20 Ω/sq with no absorbance; below thatvalue the absorbance begins to be significant. Al:ZnO is similar thoughgenerally not as good. In both cases, the transmittance is around about85-90% over most of the wavelength range.

One way to avoid the difficulties inherent in transparent oxides is touse very narrow lines of excellent metallic conductors, with wide openregions in between. To illustrate the performance of such anarchitecture, consider the resistance of an array of silver lines, 40 nmwide and 40 nm high. If 1000 such lines are placed in parallel, spaced10 μm apart (so the array is 1 cm wide), the sheet resistance would be100 Ω/sq., which is a useful range for solar cell electrodes. At thesame time, the optical transmittance would be >99% (obscured area 0.4%).

Although not limited to the following, the synthesis of silver nanorodswith diameters of about 35 nm (±5 nm) and lengths of several microns (upto 18 microns) has been described in the scientific literature (CathyMurphy et al., Nano letters, vol. 3, p. 667, 2003). The conductivity ofthese essentially single crystal nanorods is close to the bulk silverresistivity value of 1.6×10⁻⁶ Ωcm. Thus they would come within a factorof two or better of meeting the target of the hypothetical structureproposed above, if they could be connected in continuous lines anddistributed with their axes parallel.

One method of making such connections is to simply line the rods up sothat their ends are, on average, close to one another, and introduce aconducting medium in between. The conducting medium can be something ofmuch lesser conductivity, such as Al:ZnO, for example. The result is aset of very low resistance resistors, several microns long, in serieswith high resistance resistors which are in general much shorter. Theexact length of the high resistance elements depends on the method oforienting the rods. For example, flow orientation may be used: the rodsare deposited in a linear coating flow, as typical of web coating. Theextremely large aspect ratio of the rods assists in making them orientin the flow direction; polymers (later removed) can be used to refinethis order.

If the rods align predominantly into columns in the direction of flow,as is expected, then the actual resistance of a chain may be only a fewtimes the value of an ideal continuous chain. Alternatively, one may usecapping techniques to attach functional end groups to the chains. It isknown in the literature that reaction rates with ligands (typicallyorganic or organometallic molecules) are sensitive to crystal facet, sothat groups can be added preferentially to the rod ends and not to thesides. These groups can then be used to attach the rods into longchains.

Note that the actual effectiveness of such rods is greater than thesimple calculation, in that the shadowing above was calculated assuminga square cross section. In fact the rods are round, and this means thatlight striking them will bounce off in a range of directions. If theyare encased in a surrounding medium with some typical index ofrefraction in the range of 1.4-1.7, then rays with reflected anglesgreater than −55 deg. from the vertical will be totally internallyreflected at the medium—air interface, and when they come down a secondtime they are unlikely to strike another Ag nanorod, and so will enterthe solar cell absorber layer. Thus, careful choice of surroundingmedium (specifically the dielectric whose upper surface is in contactwith air) can allow up to a few percent blockage of light, and still besuperior to existing solutions with respect to optical loss. This meansthat the rods can be closer than 10 um laterally, and this increases theprobability of nanoscale separations between rod ends. The electricallyconnecting medium can be supplied by conventional means, such assputtering, or by some solution technique

In one embodiment, the ink may optionally contain a mixture oftransparent inorganic hard-to-compress (nano)powder or (nano) particles,preferably transparent conductive hard-to-compress (nano)powder, forinstance ITO or AZO, with highly-conductive metallic particles, forinstance metallic nanowires. Of course, other suitable materials asdescribed herein may also be used, singly or two or more combinations.Apart from mixing transparent metal oxide particles (spheres, flakes,tetrapods, rods, wires) with metallic nanowires (made from for instancebut not limited to silver, copper, or aluminum) other shapes thannanowires can be used as well. Additionally in an optional embodiment,it would be beneficial if the metallic particles would have a relativelylow melting point (<250 C) and/or would be malleable (meaning easy tochange form/shape upon applying external forces). Examples are gallium,indium, and typically used solder materials (see next paragraph). Thelow melting point would allow for sintering (defined as an increase incontact area between the particles in the as-deposited layer) atrelatively low temperatures. The malleability would allow forlow-temperature compression that would result in an enhanced contactarea between the particles. Note that compression for as-depositednanotubes or nanowires without added transparent inorganichard-to-compress particles only allows a very limited amount ofcompression. Over-compression of this type of layers can result in atremendous loss of transparency. Note that sintering of metallicparticles with transparent conductive particles, as suggested herein,will enhance the number of conductive pathways allowing enhanced lateralconductivity, without loosing too much transparency.

Regarding the metallic materials, one possibility would be to useparticles (or flakes) of relatively low-melting conductive material(preferably melting in a range of 150-250 C), and heat the layer (andsubstrate) to a temperature where the metallic material sinters with theother particles without damaging the underlying layers. Examples areSn—Bi, Pb—Sn, Zn—Sn, Ag—Sn, and Al—Sn. Another possibility would be touse a mixture of at least two different types of metallic particles (orflakes) where one particle has an melting point below 150 C, preferablybelow 100 C, and where the heating results in the formation of aconductive alloy (solid-solution or line-compound) with a highmelting-point, preferably far above 150 C. Examples are alloys withlow-melting materials like Ga, Cs, Rb, and Hg combined with high-meltingmaterials like Al, Cu, Fe, Ni, to form for example a Al—Gasolid-solution, Cu—Ga solid-solution or line-compound, etc.

Regarding methods, it should be understood that a two-step deposition oflow-organic-containing conductive material followed by ahigh-organic-containing mechanical stabilizer will improve both themorphology of the conductive matrix and enlarge the contact area betweenthe conductive material and the underlying surface, if compressionand/or heating need to be limited, and the top surface not necessarilyis in direct contact with conductive material. Optionally, someembodiments may reverse the sequence and apply thehigh-organic-containing material first.

The process is preferably roll-to-roll, but use of rigid substrates isnot excluded.

Non-vacuum TCE Deposition and Precursor-conversion Methods

Deposition methods include but are not limited to Roll-to-Roll AtomicLayer Deposition (R2R-ALD), Roll-to-Roll Chemical Vapor Deposition(R2R-CVD), Roll-to-Roll Lamination (R2R-lamination) of transparentconductive film, wet deposition via e.g. microgravure coating andspraying of soluble metal organic precursor. Some suitable in-line rollto roll techniques are described in U.S. patent application Ser. No.10/782,233 filed Feb. 19, 2004 and fully incorporated herein byreference for all purposes.

TCE Properties

The following are exemplary and nonlimiting. In one embodiment,thickness, may optionally be below 1 micron. Optionally,transparency >80%. Optionally, conductivity may be <200 Ohm/square, evenmore preferably <100 Ohm/square, even more preferred <50 Ohm/square.

Thermomechanical properties: Optionally, contact resistance, mayoptionally be below 1.0 mOhm-cm. The material typically has chemicalresistance to laminated materials on top of the TCE material.

TCE Three-Dimensional Structure

In one embodiment, the 3D-structure may random, organized, or parallel.

In one embodiment, the thickness of the layer may be in the range ofabout 50 nm to about 1000 nm. Optionally, the thickness of the layer maybe in the range of about 100 nm to about 500 nm. Optionally, thethickness of the layer may be in the range of about 150 nm to about 300nm.

In yet another embodiment of the present invention, the use of thenanotubes or nanowires is likely to allow thinner ZnO:Al or other TCO.This embodiment does not involve replacing ZnO:Al but using it inconjunction with the appropriate web-like conductor. ZnO:Al or othertransparent conductive material as thin as about 100 nm might be enoughto stop shunting. Optionally, the layer of ZnO:Al or other transparentconductive material may be about 100 to about 200 nm in thickness.Optionally, the layer of ZnO:Al or other transparent conductive materialmay be about 100 to about 500 nm in thickness. CNTs or metal nanowiresmay be formed on top of this layer or in this layer. Instead they areagglomerated and form particle monolayers. This saves time in sputteringand materials used for ZnO:Al. In one embodiment, the ZnO:Al may bebelow the CNT or other web like layer. In one embodiment, the ZnO:Al maybe above the CNT or other web like layer.

Adhesion with Minimal Loss of Contact Resistance

Where various materials, like metals, provide both low volumeresistivity and low contact resistance, typically these materials havelow optical transparency.

CONDUCTIVE PRECURSOR IMPROVING VOLUME RESISTIVITY UPON CONVERSION. Byway of example and not limitation, this may include TiO2 via metaldeposition followed by oxygen plasma treatment. Optionally, this mayinclude TiO2 core, Ti shell, oxygen plasma treatment. Optionally, thismay include ZnO:Al core, Ti shell, oxygen plasma treatment. Optionally,this may include precursor to metal, e.g. metal organic compounds thatdecompose upon heat.

Conductive Filler Improving Volume Resistivity Upon Filling

This may include use of one or more of the following: fillers; wires,3D-structures, cables, spheres, tetrapods, and/or combinations of theforegoing.

HYBRIDS OF THE FOREGOING may include multiple depositions, conversions,materials, timing (prior to, during, or after) for any of the stepsherein.

Transparent Conductive Electrode Film Materials

Optionally, the transparent conductive electrode film may includematerials; metals, conductive oxides, conductive nitrides, conjugatedmolecules, conjugated polymers, fullerenes, TCO particles, dopedsemiconductor particles (spheres, tetrapods, rods, wires), SOLDERS,Ga-AMALGAMS, TCO: AZO, GZO, BZO, ITO and the like.

Optionally, the material may be organo-metallic precursor containingeither Al, Ga, and/or B & TCO particles.

Optionally, the method may include depositing molecularly dissolved orsolid particles of organo-metallic precursors containing Zn, and dopantslike Al, B, Ga, and/or other dopants, graphite sheets, the like, and/orcombinations of the foregoing. Any of the techniques may be combined insingle or multiple combinations of any other technique described herein.

Optionally, the method may include using ultrathin layer of metal (usingtechniques such as but not limited to electroless deposition, ALD,thermal decomposition of a solution-deposited soluble metal precursor,or the like), like Ti, Zn, Zr or the like providing metal contactbetween silver fingers and TCO, but oxidizing the unexposed metal viae.g. an atmospheric oxygen plasma will improve transparency; Cu, Sn, Hf,Ru.

Optionally, the method may include integratingtraces/grids/fingers/lines with underlying TCE.

Typically in thin-film PV, Liquid Crystal Displays, Light emittingdiodes, and other opto-electronic applications, the transparentconductive layer is a transparent conductive oxide deposited in slowexpensive vacuum equipment. New lower-cost materials and lower-costdeposition methods that have been developed are solution-deposition ofcarbon nanotubes or metal nanowires that both result in a more or lessrandom percolating network of conductive tubes/wires. Typically thesenetworks are stabilized by co-deposition or over-coating with a polymermatrix. Relying on percolating networks, and therefore a combination ofhopping conduction and conduction through the wires/tubes limits theconductivity and/or the transparency especially when both thetransparency and the conductivity need to be increased simultaneously.

The embodiments described here allow not only for an alternative method,but also for a better combined performance from both an electrical andoptical point-of-view, meaning the invention allows for more facile andindependent adjustment of conductivity (in all three dimensions) andoptical properties (minimize optical losses).

The preferred method of creating a low-temperature curable transparentconductor is by solution deposition of transparent block-co-polymerscombined with simultaneous or subsequent deposition of metal precursorsthat will fill the pores with a metal organized 3D-network. Depending onthe type of block-co-polymers and chemistry used for the metallizationof the porous highly organized block-co-polymer network, the filling ofthe pores can either be performed on top of the stack/substrate in thefinal product, or separately, and subsequently be transferred as amechanically stable film to the final product followed by lamination.Filling the pores of the block-co-polymer structure can be performed byelectro-deposition, electro-less deposition, like chemical bathdeposition, chemical surface deposition, and horizontal bath deposition,spraying, solution coating, solution printing, etc. Similar precursorscan be used as applied for wet chemical synthesis of metal nano-powders.Other deposition methods that can be used to fill the polymer networkare atomic layer deposition, chemical vapor deposition, and the like.

Apart from filling the polymer network with metallic precursors thatconvert to metal, the porous polymer network can be filled with carbonblack, fullerenes, metal nanopowder, transparent conductive oxideprecursors (sol-gel), TCO nanopowder, or filled by vacuum deposition ofTCO (or metal). The latter two examples (using vacuum deposition to filla porous polymer network) would be particularly interesting whentransferring a mechanically stable polymer film filled with conductivematerial to the final product, where the final product cannot withstandhigh temperatures and the curing of the polymer network with/withoutcuring of the conductive network requires high temperatures.

One embodiment of the invention may comprise of high-efficiencythin-film solar cells based on polycrystalline CIGS (copper indiumgallium di-selenide, but not excluding any other of the IB, IIIA, VIAelements like e.g. aluminum, and sulfur) are typically made with atransparent conductive oxide on top requiring additional conductivepatterns to collect the current with minimal resistive losses. Loweringthe cost of the deposition of these patterns is required to minimize theoverall cost of the solar panels.

One major challenge to make highly-conductive patterns viasolution-deposition is to be able to formulate an ink (slurry, paste,dispersion, emulsion, paint) that allows solution-deposition ofconductive materials onto a substrate without the negative influenceorganic additives might have on the contact resistance to the substrate(being the transparent conductive oxide) and conductivity within thebulk of the pattern, since typically solution-deposited patterns rely onhopping conductance between particles thereby making the conductivity(but also the contact area for the conductive material in the two-phasepatterns with the substrate) very sensitive to the morphology of thetwo-phase system of insulating-organic and conductive material.Additionally, subsequent heating (temperature and time) to mechanicallystabilize these patterns and/or improve on contact resistance and/orimprove on bulk-conductivity needs to be limited not to damage theunderlying layers. Furthermore, the difference in the coefficient ofthermal expansion between organic additives and the conductive componentin the ink is typically large, which might cause difficulties duringheating after solution-deposition or might limit the stability of thesepatterns over time.

In order to overcome the difficulties with typical inks used forsolution-deposition, a new material and method is proposed in thisinvention disclosure.

Regarding materials: One embodiment uses particles (or flakes) ofrelatively low-melting conductive material (preferably melting in arange of 150-250 C), and heat the pattern (and substrate) to atemperature where the conductive material sinters without damaging theunderlying layers. Examples include but are not limited to Sn—Bi, Pb—Sn,Zn—Sn, Ag—Sn, and Al—Sn. Another possibility would be to use a mixtureof at least two different types of particles (or flakes) where oneparticle has an melting point below 150 C, preferably below 100 C, andwhere the heating results in the formation of a conductive alloy(solid-solution or line-compound) with a high melting-point, preferablyfar above 150 C. Examples are alloys with low-melting materials like Ga,Cs, Rb, and Hg combined with high-melting materials like Al, Cu, Fe, Ni,to form for example a Al—Ga solid-solution, Cu—Ga solid-solution orline-compound, etc.

Regarding method, one embodiment may use a two-step deposition oflow-organic-containing conductive material followed by ahigh-organic-containing mechanical stabilizer will improve both themorphology of the conductive matrix and enlarge the contact area betweenthe conductive material and the substrate (TCO).

The process is preferably roll-to-roll, but use of rigid substrates isnot excluded.

Photovoltaic Device Chemistry

A variety of different chemistries to arrive at a desired semiconductorfilm for the absorber layer and the solution deposited transparentconductor is not limited to any particular type of solar cell orabsorber layer. Although not limited to the following, an active layerfor a photovoltaic device may be fabricated by formulating an ink ofspherical and/or non-spherical particles each containing at least oneelement from groups IB, IIIA and/or VIA, coating a substrate with theink to form a precursor layer, and heating the precursor layer to form adense film. By way of nonlimiting example, the particles themselves maybe elemental particles or alloy particles. In some embodiments, theprecursor layer forms the desired group IB-IIIA-VIA compound in a onestep process. In other embodiments, a two step process is used wherein adense film is formed and then further processed in a suitable atmosphereto form the desired group IB-IIIA-VIA compound. It should be understoodthat chemical reduction and/or densification of the precursor layer maynot be needed in some embodiments, particularly if the precursormaterials are oxygen-free or substantially oxygen free. Thus, a firstheating step of two sequential heating steps may optionally be skippedif the particles are processed air-free and are oxygen-free. Theresulting group IB-IIIA-VIA compound for either a one step or a two stepprocess is preferably a compound of Cu, In, Ga and selenium (Se) and/orsulfur S of the form CuIn_((1-x))Ga_(x)S_(2(1-y))Se_(2y), where 0≦x≦1and 0≦y≦1. Optionally, the resulting group IB-IIIA-VIA compound may be acompound of Cu, In, Ga and selenium (Se) and/or sulfur S of the formCu_(z)In_((1-x))Ga_(x)S_(2(1-y))Se_(2y), where 0.5≦z≦1.5, 0≦x≦1.0 and0≦y≦1.0. Optionally, the resulting group IB-IIIA-VIA thin-film may be amixture of compounds of Cu, In, Ga and selenium (Se) and/or sulfur S ofthe form Cu_(z)In_((1-x))Ga_(x)S_((2+w)(1-y))Se_((2+w)y), where0.5≦z≦1.5, 0≦x<1.0, 0≦y≦1.0, and 0≦w≦0.5.

It should also be understood that group IB, IIIA, and VIA elements otherthan Cu, In, Ga, Se, and S may be included in the description of theIB-IIIA-VIA materials described herein, and that the use of a hyphen(“—”e.g., in Cu—Se or Cu—In—Se) does not indicate a compound, but ratherindicates a coexisting mixture of the elements joined by the hyphen. Itis also understood that group IB is sometimes referred to as group 11,group IIIA is sometimes referred to as group 13 and group VIA issometimes referred to as group 16. Furthermore, elements of group VIA(16) are sometimes referred to as chalcogens. Where several elements canbe combined with or substituted for each other, such as In and Ga, orSe, and S, in embodiments of the present invention, it is not uncommonin this art to include in a set of parentheses those elements that canbe combined or interchanged, such as (In, Ga) or (Se, S). Thedescriptions in this specification sometimes use this convenience.Finally, also for convenience, the elements are discussed with theircommonly accepted chemical symbols. Group IB elements suitable for usein the method of this invention include copper (Cu), silver (Ag), andgold (Au). Preferably the group IB element is copper (Cu). Group IIIAelements suitable for use in the method of this invention includegallium (Ga), indium (In), aluminum (Al), and thallium (Tl). Preferablythe group IIIA element is gallium (Ga) and/or indium (In). Group VIAelements of interest include selenium (Se), sulfur (S), and tellurium(Te), and preferably the group VIA element is either Se and/or S. Itshould be understood that mixtures such as, but not limited to, alloys,solid solutions, and compounds of any of the above can also be used. Theshapes of the solid particles may be any of those described herein.

High Efficiency Cell Configuration

It should be understood that the device manufactured as shown in FIG. 1and the above paragraphs may be suitable for use in a high efficiencycell configuration as detailed below in FIG. 3A. FIG. 3A illustrates anarray 100 of optoelectronic devices according to an embodiment of thepresent invention. In some embodiments, this may be considered a seriesinterconnection in an array 100 of optoelectronic devices. The array 100includes a first device module 101 and a second device module 111. Thedevice modules 101, 111 may be photovoltaic devices, such as solarcells, or light-emitting devices, such as light-emitting diodes. In apreferred embodiment, the device modules 101, 111 are solar cells. Thefirst and second device modules 101, 111 are attached to an insulatingcarrier substrate 103, which may be made of a plastic material such aspolyethylene terephthalate (PET), e.g., about 50 microns thick. Thecarrier substrate 103 may, in turn, be attached to a thicker structuralmembrane 105, e.g., made of a polymeric roofing membrane material suchas thermoplastic polyolefin (TPO) or ethylene propylene diene monomer(EPDM), to facilitate installing the array 100 on an outdoor locationsuch as a roof.

By way of nonlimiting example, the device modules 101, 111, which may beabout 4 inches in length and 12 inches wide, may be cut from a muchlonger sheet containing several layers that are laminated together. Eachdevice module 101, 111 generally includes a device layer 102, 112 incontact with a bottom electrode 104, 114 and an insulating layer 106,116 between the bottom electrode 104, 114 and a conductive back plane108, 118. It should be understood that in some embodiments of thepresent invention, the back plane 108, 118 may be described as abackside top electrode 108, 118. The bottom electrodes 104, 114,insulating layers 106, 116 and back planes 108, 118 for substrates S₁,S₂ support the device layers 102, 112

In contrast to prior art cells, where the substrates are formed bydepositing thin metal layers on an insulating substrate, embodiments ofthe present invention utilize substrates S₁, S₂ based on flexible bulkconducting materials, such as foils. Although bulk materials such asfoils are thicker than prior art vacuum deposited metal layers they canalso be cheaper, more readily available and easier to work with.Preferably, at least the bottom electrode 104, 114 is made of a metalfoil, such as aluminum foil. Alternatively, copper, stainless steel,titanium, molybdenum or other suitable metal foils may be used. By wayof example, the bottom electrodes 104, 114 and back planes 108, 118 maybe made of aluminum foil about 1 micron to about 200 microns thick,preferably about 25microns to about 100 microns thick; the insulatinglayers 106, 116 may be made of a plastic foil material, such aspolyethylene terephthalate (PET) about 1 micron to about 200 micronsthick, preferably about 10 microns to about 50 microns thick. In oneembodiment, among others, the bottom electrode 104,114, insulating layer106, 116 and back plane 108, 118 are laminated together to form thestarting substrates S₁, S₂. Although foils may be used for both thebottom electrode 104, 114 and the back plane 108, 118 it is alsopossible to use a mesh grid on the back of the insulating layer 106, 116as a back plane. Such a grid may be printed onto the back of theinsulating layer 106, 116 using a conductive ink or paint. One example,among others, of a suitable conductive paint or ink is Dow Corning®PI-2000 Highly Conductive Silver Ink available from Dow CorningCorporation of Midland Michigan. Dow Corning® is a registered trademarkof Dow Corning Corporation of Midland Michigan. Furthermore, theinsulating layer 106, 116 may be formed by anodizing a surface of a foilused for the bottom electrode 104, 114 or back plane 108, 118 or both,or by applying an insulating coating by spraying, coating, or printingtechniques known in the art.

The device layers 102, 112 generally include an active layer 107disposed between a transparent conductive layer 109 and the bottomelectrode 104. It should be understood that the transparent conductivelayer 109 may be any of the solution deposited transparent conductorsdescribed herein. Optionally, the transparent conductor layer 109 may bemetal rod, nanotube, web-like, or mesh-type electrode with sufficientspacing between elements so as to be substantially transparent in aspectral range from about 400 nm to about 1100 nm while still capable ofcarrying an electrical charge laterally. They may be with or without abinder. By way of example, the device layers 102, 112 may be about 2microns thick. At least the first device 101 includes one or moreelectrical contacts 120 between the transparent conducting layer 109 andthe back plane 108. The electrical contacts 120 are formed through thetransparent conducting layer 109, the active layer 107, the bottomelectrode 104 and the insulating layer 106. The electrical contacts 120provide an electrically conductive path between the transparentconducting layer 109 and the back plane 108. The electrical contacts 120are electrically isolated from the active layer 107, the bottomelectrode 104 and the insulating layer 106.

The contacts 120 may each include a via formed through the active layer107, the transparent conducting layer 109, the bottom electrode 104 andthe insulating layer 106. Each via may be about 0.1 millimeters to about1.5 millimeters, preferably 0.5 millimeters to about 1 millimeter indiameter. The vias may be formed by punching or by drilling, for exampleby mechanical, laser or electron beam drilling, or by a combination ofthese techniques. An insulating material 122 coats sidewalls of the viasuch that a channel is formed through the insulating material 122 to theback plane 108. The insulating material 122 may have a thickness betweenabout 1 micron and about 200 microns, preferably between about 10microns and about 200 microns.

The insulating material 122 should preferably be at least 10 micronsthick to ensure complete coverage of the exposed conductive surfacesbehind it. The insulating material 122 may be formed by a variety ofprinting techniques, including for example inkjet printing or dispensingthrough an annular nozzle. A plug 124 made of an electrically conductivematerial at least partially fills the channel and makes electricalcontact between the transparent conducting layer 109 and the back plane108. The electrically conductive material may similarly be printed. Asuitable material and method, for example, is inkjet printing of solder(called “solderjet” by Microfab, Inc., Plano, Tex., which sellsequipment useful for this purpose). Printing of conductive adhesivematerials known in the art for electronics packaging may also be used,provided time is allowed subsequently for solvent removal and curing.The plug 124 may have a diameter between about 5 microns and about 500microns, preferably between about 25 and about 100 microns.

By way of nonlimiting example, in other embodiments, the device layers102, 112 may be about 2 microns thick, the bottom electrodes 104, 114may be made of aluminum foil about 100 microns thick; the insulatinglayers 106, 116 may be made of a plastic material, such as polyethyleneterephthalate (PET) about 25 microns thick; and the backside topelectrodes 108, 118 may be made of aluminum foil about 25 microns thick.The device layers 102, 112 may include an active layer 107 disposedbetween a transparent conductive layer 109 and the bottom electrode 104.In such an embodiment, at least the first device 101 includes one ormore electrical contacts 120 between the transparent conducting layer109 and the backside top electrode 108. The electrical contacts 120 areformed through the transparent conducting layer 109, the active layer107, the bottom electrode 104 and the insulating layer 106. Theelectrical contacts 120 provide an electrically conductive path betweenthe transparent conducting layer 109 and the backside top electrode 108.The electrical contacts 120 are electrically isolated from the activelayer 107, the bottom electrode 104 and the insulating layer 106.

The formation of good contacts between the conductive plug 124 and thesubstrate 108 may be assisted by the use of other interface-formingtechniques such as ultrasonic welding. An example of a useful techniqueis the formation of gold stud-bumps, as described for example by J. JayWimer in “3-D Chip Scale with Lead-Free Processes” in SemiconductorInternational, Oct. 1, 2003, which is incorporated herein by reference.Ordinary solders or conductive inks or adhesives may be printed on topof the stud bump.

In forming the vias, it is important to avoid making shortingconnections between the top electrode 109 and the bottom electrode 104.Therefore, mechanical cutting techniques such as drilling or punchingmay be advantageously supplemented by laser ablative removal of a smallvolume of material near the lip of the via, a few microns deep and a fewmicrons wide. Alternatively, a chemical etching process may be used toremove the transparent conductor over a diameter slightly greater thanthe via. The etching can be localized, e.g., by printing drops ofetchant in the appropriate places using inkjet printing or stencilprinting.

A further method for avoiding shorts involves deposition of a thin layerof insulating material on top of the active layer 107 prior todeposition of the transparent conducting layer 109. This insulatinglayer is preferably several microns thick, and may be in the range of 1to 100 microns. Since it is deposited only over the area where a via isto be formed (and slightly beyond the borders of the via), its presencedoes not interfere with the operation of the optoelectronic device. Insome embodiments of the present invention, the layer may be similar tostructures described in U.S. patent application Ser. No. 10/810,072 toKarl Pichler, filed Mar. 25, 2004, which is hereby incorporated byreference. When a hole is drilled or punched through this structure,there is a layer of insulator between the transparent conducting layer109 and the bottom electrode 104 which may be relatively thick comparedto these layers and to the precision of mechanical cutting processes, sothat no short can occur.

The material for this layer can be any convenient insulator, preferablyone that can be digitally (e.g. inkjet) printed. Thermoplastic polymerssuch as Nylon PA6 (melting point (m.p.) 223° C..), acetal (m.p. 165°C..), PBT (structurally similar to PET but with a butyl group replacingthe ethyl group) (m.p. 217° C..), and polypropylene (m.p.165° C..), areexamples which by no means exhaust the list of useful materials. Thesematerials may also be used for the insulating layer 122. While inkjetprinting is a desirable way to form the insulator islands, other methodsof printing or deposition (including conventional photolithography) arealso within the scope of the invention.

In forming the vias, it is useful to fabricate the optoelectronic devicein at least two initially separate elements, with one comprised of theinsulating layer 106, the bottom electrode 104 and the layers 102 aboveit, and the second comprised of the back plane 108. These two elementsare then laminated together after the vias have been formed through thecomposite structure 106/104/102, but before the vias are filled. Afterthis lamination and via formation, the back plane 108 is laminated tothe composite, and the vias are filled as described above.

Although jet-printed solders or conductive adhesives comprise usefulmaterials for forming the conductive via plug 124, it is also possibleto form this plug by mechanical means. Thus, for example, a wire ofsuitable diameter may be placed in the via, forced into contact with theback plane 108, and cut off at the desired height to form the plug 124,in a manner analogous to the formation of gold stud bumps. Alternativelya pre-formed pin of this size can be placed into the hole by a roboticarm. Such pins or wires can be held in place, and their electricalconnection to the substrate assisted or assured, by the printing of avery thin layer of conductive adhesive prior to placement of the pin. Inthis way the problem of long drying time for a thick plug of conductiveadhesive is eliminated. The pin can have tips or serrations on it whichpunch slightly into the back plane 108, further assisting contact. Suchpins may be provided with insulation already present, as in the case ofinsulated wire or coated wire (e.g. by vapor deposition or oxidation).They can be placed in the via before the application of the insulatingmaterial, making it easier to introduce this material.

If the pin is made of a suitably hard metal, and has a slightly taperedtip, it may be used to form the via during the punching step. Instead ofusing a punch or drill, the pin is inserted into the composite106/104/102, to a depth such that the tip just penetrates the bottom;then when the substrate 108 is laminated to this composite, the tippenetrates slightly into it and forms a good contact. These pins may beinjected into the unpunched substrate by, for example, mechanicalpressure or air pressure directed through a tube into which the pin justfits.

The first device module 101 may be attached to the carrier substrate 103such that the back plane 108 makes electrical contact with the thinconducting layer 128 while leaving a portion of the thin conductinglayer 128 exposed. Electrical contact may then be made between theexposed portion of the thin conducting layer 128 and the exposed portionof the bottom electrode 114 of the second device module 111. Forexample, a bump of conductive material 129 (e.g., more conductiveadhesive) may be placed on the thin conducting layer 128 at a locationaligned with the exposed portion of the bottom electrode 114. The bumpof conductive material 129 is sufficiently tall as to make contact withthe exposed portion of the bottom electrode 114 when the second devicemodule 111 is attached to the carrier substrate. The dimensions of thenotches 117, 119 may be chosen so that there is essentially nopossibility that the thin conducting layer 128 will make undesiredcontact with the back plane 118 of the second device module 111. Forexample, the edge of the bottom electrode 114 may be cut back withrespect to the insulating layer 116 by an amount of cutback CB₁ of about400 microns. The back plane 118 may be cut back with respect to theinsulating layer 116 by an amount CB₂ that is significantly larger thanCB₁. Optionally, the backside conductor or backplane 108 may be extendedas shown by phantom section 131 to extend to be positioned below thebottom electrode 114 of an adjacent cell. In one embodiment, the twolayers 131 and 114 may be connected together by a variety of methodssuch as but not limited to ultrasonic welding, laser welding, soldering,or other techniques to create an electrical connection. The layer 131may be bent or shaped to better engage the section 114. Some embodimentsmay have holes, openings, or cutaways in the layer 131 to facilitateattachment.

The device layers 102, 112 are preferably of a type that can bemanufactured on a large scale, e.g., in a roll-to-roll processingsystem. There are a large number of different types of devicearchitectures that may be used in the device layers 102, 112. By way ofexample, and without loss of generality, the inset in FIG. 1A shows thestructure of a CIGS active layer 107 and associated layers in the devicelayer 102. By way of example, the active layer 107 may include anabsorber layer 130 based on materials containing elements of groups IB,IIIA and VIA. Preferably, the absorber layer 130 includes copper (Cu) asthe group IB, Gallium (Ga) and/or Indium (In) and/or Aluminum as groupMA elements and Selenium (Se) and/or Sulfur (S) as group VIA elements.Examples of such materials (sometimes referred to as CIGS materials) aredescribed in U.S. Pat. No. 6,268,014, issued to Eberspacher et al onJul. 31, 2001, and US Patent Application Publication No. US 2004-0219730A1 to Bulent Basol, published Nov. 4, 2004, both of which areincorporated herein by reference. A window layer 132 is typically usedas a junction partner between the absorber layer 130 and the transparentconducting layer 109. By way of example, the window layer 132 mayinclude cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc selenide(ZnSe) or some combination of two or more of these. Layers of thesematerials may be deposited, e.g., by chemical bath deposition orchemical surface deposition, to a thickness of about 50 nm to about 100nm. A contact layer 134 of a metal different from the bottom electrodemay be disposed between the bottom electrode 104 and the absorber layer130 to inhibit diffusion of metal from the bottom electrode 104. Forexample, if the bottom electrode 104 is made of aluminum, the contactlayer 134 may be a layer of molybdenum.

Although CIGS solar cells are described for the purposes of example,those of skill in the art will recognize that embodiments of the seriesinterconnection technique can be applied to almost any type of solarcell architecture. Examples of such solar cells include, but are notlimited to: cells based on amorphous silicon, Graetzel cell architecture(in which an optically transparent film comprised of titanium dioxideparticles a few nanometers in size is coated with a monolayer of chargetransfer dye to sensitize the film for light harvesting), a nanostructured layer having an inorganic porous semiconductor template withpores filled by an organic semiconductor material (see e.g., US PatentApplication Publication US 2005-0121068 Al, which is incorporated hereinby reference), a polymer/blend cell architecture, organic dyes, and/orC₆₀ molecules, and/or other small molecules, micro-crystalline siliconcell architecture, randomly placed nanorods and/or tetrapods ofinorganic materials dispersed in an organic matrix, quantum dot-basedcells, or combinations of the above. Furthermore, embodiments of theseries interconnection technique described herein can be used withoptoelectronic devices other than solar cells.

While the invention has been described and illustrated with reference tocertain particular embodiments thereof, those skilled in the art willappreciate that various adaptations, changes, modifications,substitutions, deletions, or additions of procedures and protocols maybe made without departing from the spirit and scope of the invention.For example, with any of the above embodiments, carbon nanotubes may beused alone or in combination with an adjacent layer to form thetransparent electrode and minimize shunting. It should be understoodthat the embodiments herein may be suitable for addressing web-likeconductors made of other materials such as noble metal based or noblemetal nanoarchitected webs or meshes (or their alloys) and are notlimited to the carbon nanotubes. It should be understood that the carbonnanotubes layer may be deposited in one step and a binder applied in asecond step. Optionally, the binder and web-like conductors are appliedsimultaneously. In some embodiments, the web-like conductors aresuspended in dispersion with a layer of material (such as for thejunction partner or the transparent conductor) and solution depositedsimultaneously. Some embodiments may have a layer of web-liketransparent conductor and then a layer of ZnO on top. Optionally, thepositions may be reversed with the ZnO on the bottom and the web-liketransparent conductor on top. As mentioned, the use of ZnO is purelyexemplary and other transparent materials may be used.

Furthermore, those of skill in the art will recognize that any of theembodiments of the present invention can be applied to almost any typeof solar cell material and/or architecture. For example, the absorberlayer in the solar cell may be an absorber layer comprised of silicon,amorphous silicon, organic oligomers or polymers (for organic solarcells), bi-layers or interpenetrating layers or inorganic and organicmaterials (for hybrid organic/inorganic solar cells), dye-sensitizedtitania nanoparticles in a liquid or gel-based electrolyte (for Graetzelcells in which an optically transparent film comprised of titaniumdioxide particles a few nanometers in size is coated with a monolayer ofcharge transfer dye to sensitize the film for light harvesting),copper-indium-gallium-selenium (for CIGS solar cells), CdSe, CdTe,Cu(In,Ga)(S,Se)₂, Cu(In,Ga,Al)(S,Se,Te)₂, and/or combinations of theabove, where the active materials are present in any of several formsincluding but not limited to bulk materials, micro-particles,nano-particles, or quantum dots. The CIGS cells may be formed by vacuumor non-vacuum processes. The processes may be one stage, two stage, ormulti-stage CIGS processing techniques. Optionally, some embodiments maybe from a group IB-IIB-IVA-VIA compound absorber layer. Additionally,other possible absorber layers may be based on amorphous silicon (dopedor undoped), a nanostructured layer having an inorganic poroussemiconductor template with pores filled by an organic semiconductormaterial (see e.g., US Patent Application Publication US 2005-0121068A1, which is incorporated herein by reference), a polymer/blend cellarchitecture, organic dyes, and/or C₆₀ molecules, and/or other smallmolecules, micro-crystalline silicon cell architecture, randomly placednanorods and/or tetrapods of inorganic materials dispersed in an organicmatrix, quantum dot-based cells, or combinations of the above. Many ofthese types of cells can be fabricated on flexible substrates.

Additionally, concentrations, amounts, and other numerical data may bepresented herein in a range format. It is to be understood that suchrange format is used merely for convenience and brevity and should beinterpreted flexibly to include not only the numerical values explicitlyrecited as the limits of the range, but also to include all theindividual numerical values or sub-ranges encompassed within that rangeas if each numerical value and sub-range is explicitly recited. Forexample, a thickness range of about 1 nm to about 200 nm should beinterpreted to include not only the explicitly recited limits of about 1nm and about 200 nm, but also to include individual sizes such as butnot limited to 2 nm, 3 nm, 4 nm, and sub-ranges such as 10 nm to 50 nm,20 nm to 100 nm, etc.

The publications discussed or cited herein are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.Further, the dates of publication provided may be different from theactual publication dates which may need to be independently confirmed.All publications mentioned herein are incorporated herein by referenceto disclose and describe the structures and/or methods in connectionwith which the publications are cited. For example, U.S. Patentapplications Ser. No. 60/909,357 filed Mar. 30, 2007 and 60/913,260filed Apr. 20, 2007 are fully incorporated herein by reference for allpurposes.

While the above is a complete description of the preferred embodiment ofthe present invention, it is possible to use various alternatives,modifications and equivalents. Therefore, the scope of the presentinvention should be determined not with reference to the abovedescription but should, instead, be determined with reference to theappended claims, along with their full scope of equivalents. Anyfeature, whether preferred or not, may be combined with any otherfeature, whether preferred or not. In the claims that follow, theindefinite article “A”, or “An” refers to a quantity of one or more ofthe item following the article, except where expressly stated otherwise.The appended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase “means for.”

What is claimed is:
 1. A method comprising: processing a precursor layerin one or more steps to form a photovoltaic absorber layer; depositing atransparent material layer loaded with conductive nanopowder ornanoparticles.
 2. The method of claim 1 further comprising forming asacrificial layer coupled to the transparent material layer.
 3. Themethod of claim 1 wherein the transparent material layer includes aweb-like layer of conductive material comprises a plurality of metallicnanowires.
 4. The method of claim 1 wherein the photovoltaic absorberlayer comprises of a group IB-IIIA-VIA material.
 5. The method of claim1 wherein the photovoltaic absorber layer prior to deposition of asmoothing layer has peaks between about 10 to about 1000 nm.
 6. Themethod of claim 1 wherein the photovoltaic absorber layer prior todeposition of a smoothing layer has peaks between about 10 to about 500nm.
 7. The method of claim 1 wherein the photovoltaic absorber layerprior to deposition of a smoothing layer has peaks between about 10 toabout 100 nm.
 8. The method of claim 1, further comprising depositing asmoothing layer on the photovoltaic absorber layer, wherein thesmoothing layer comprises an electrically conductive material.
 9. Themethod of claim 8 wherein the smoothing layer comprises one or more ofthe following: sol gel TCO or TCO particles.
 10. The method of claim 8wherein the smoothing layer comprises ZnO:Al of a thickness of about 100nm or less.
 11. The method of claim 8 wherein the smoothing layercomprises ZnO:Al of a thickness of about 150 nm or less.
 12. The methodof claim 8 wherein the smoothing layer comprises ZnO:Al of a thicknessof about 200 nm or less.
 13. The method of claim 8 wherein the smoothinglayer and web layer have substantially the same thickness.
 14. Themethod of claim 8 wherein the smoothing layer comprises a electricallyconductive leveling layer.
 15. A method comprising: processing aprecursor layer in one or more steps to form a photovoltaic absorberlayer; depositing a smoothing, insulating layer to fill gaps anddepression in the absorber layer to reduce a roughness of the absorberlayer.
 16. The method of claim 15 wherein the smoothing insulator issufficient to cover all peaks of the rough absorber, but sufficientlythin to allow electrons to pass out of the absorber layer.
 17. Themethod of claim 15 wherein the smoothing insulator conformally coversall peaks in the rough absorber.
 18. The method of claim 15 furthercomprising creating the absorber layer by processing the precursor layerinto a solid film and then thermally reacting the solid film in anatmosphere containing at least an element of Group VIA of the PeriodicTable to form the photovoltaic absorber layer.
 19. The method of claim15 further comprising creating the absorber layer by thermal reaction ofthe precursor layer in an atmosphere containing at least an element ofGroup VIA of the Periodic Table to form the photovoltaic absorber layer.20. The method of claim 15 wherein Group IB and/or IIIA hydroxidecomprises indium-gallium hydroxide.